A creature that’s put onto the battlefield blocking isn’t affected by requirements or restrictions that apply to the declaration of blockers.
September 30, 2016 · Kaladesh
rule 509.4b not present in this release
May 1, 2014 · Journey into Nyx
An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn’t already blocking that attacking creature at that time. It won’t trigger if the creature is put onto the battlefield blocking.
February 1, 2014 · Born of the Gods
An ability that reads "Whenever [this creature] blocks a creature, . . ." triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn’t already blocking that attacking creature at that time. It won’t trigger if the creature is put onto the battlefield blocking.
September 30, 2011 · Innistrad
An ability that reads "Whenever [this creature] blocks a creature, . . ." triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn't already blocking that attacking creature at that time. It won't trigger if the creature is put onto the battlefield blocking.
June 17, 2011 · Commander 2011
An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each
April 1, 2011 · Duel Decks: Knights vs. Dragons
An ability that reads "Whenever [this creature] blocks a creature, . . ." triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn't already blocking that attacking creature at that time. It won't trigger if the creature is put onto the battlefield blocking.
February 4, 2011 · Mirrodin Besieged
An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn’t already blocking that attacking creature at that time. It won’t trigger if the creature is put onto the battlefield blocking.
October 1, 2010 · Scars of Mirrodin
An ability that reads “Whenever [this creature] blocks a creature, . . .” triggers once for each
February 1, 2010 · Worldwake
An ability that reads "Whenever [this creature] blocks a creature, . . ." triggers once for each attacking creature the creature with the ability blocks. It triggers if the creature is declared as a blocker. It will also trigger if an effect causes that creature to block an attacking creature, but only if it wasn't already blocking that attacking creature at that time. It won't trigger if the creature is put onto the battlefield blocking.